Why is vital? If a unidirectional bezel is knocked and unintentionally moved with a dive, it could only display an extended underwater time. Meaning the wearer will overestimate rather than undervalue immersion time, As well as in situations that have to have decompression stops, This may be a make-it-or-split-it factor https://www.watchesworld.com/frozen-in-time-luxury-watches-for-winter/